Reasons to allow null as a HashMap key?

Question

Why did Java decide to allow "null" as a key in HashMap? Since we know that hashcode () cannot be calculated by a zero value. Is there any design consideration for this solution to allow a null key in a HashMap that is not in the Hashtable (other than accounting as the default)?

Answer 1

From JDK 1.2 Summary of the Java Collections API (I don’t know where to find the official version on the Oracle website):

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